Tank mounted rotary compressor

ABSTRACT

A rotary compressor having a drive motor and a single pressure vessel, wherein the pressure vessel acts both as a gas oil separator and as a compressed gas storage tank. The compressor employs a control for valves in order to close off the supply of oil and gas from entering the compressor prior to cessation of the compressor&#39;s rotation.

The present invention relates to improvements in rotary compressorsystems. Rotary compressor systems include screw compressors utilisingintermeshing rotors, vane and scroll type compressors.

Conveniently, rotary compressor systems comprise a compressor unit, adrive motor drivingly coupled to the compressor unit to drive same, aseparator vessel defining a volume containing a supply of lubricatingliquid (hereinafter called "oil") and arranged to receive a mixture ofcompressed gas and liquid from the compressor unit, a filter elementthrough which compressed gas flows to a clean compressed gas storagetank, an oil filter and oil cooling device through which oil passes in areturn line from the separator vessel to an inlet region of thecompressor unit, and appropriate piping and valving linking the systemtogether. Various improvements have been proposed to such systems toimprove performance, limit componentry to decrease manufacturing costsand to decrease package sizes, however, such systems still remainrelatively complex with package sizes larger than equivalentreciprocating compressor systems, particularly in smaller capacitymachines.

Such systems have also always had competing design interests. Forexample, to reduce package sizes, it is desirable to, reduce thephysical size of the larger volume components such as the separatorvessel and the gas storage tank. However, to improve capability of themachine to work longer between service periods to replace the oil, it isdesirable for the separator vessel to be as large as possible so thatthe volume of oil used in the system can also be as large as possible.Moreover, with systems using minimum pressure valves (mpv) to maintain aminimum pressure in the separator so as to allow oil circulation back tothe compressor unit by pressure differential, it is generally desirableto keep the separator volume below a certain level so as to prevent toomuch of a delay at start up before the minimum pressure is achieved sothat lubricating oil can be returned to the compressor unit. The problemis exacerbated by the oil desirably entering the compressor unit at apressure greater than atmospheric pressure (say 2.5 atmospheres) so asnot to obstruct suction volumes of air through the compressor unit.Thus, the minimum pressure level needs to be above this level (say 3.5atmospheres) to create the necessary pressure differential. Thus, if theseparator volume is too large the screw rotors may seize beforelubricating oil starts to flow. Opposed to this, it is also desirable tohave a separator vessel volume as large as possible so that it can copewith oil foaming (which occurs during certain stages of systemoperation) without having the foaming oil flowing into bulk contact withthe final filter element. The tendency has, however, been to designcompressor systems with ever decreasing sized separator vessel volumessometimes with attempts to solve the aforementioned oil volume andfoaming problems by other techniques. It still remains, however, adesirable attribute that the separator vessel be as large as possible toallow use of increased oil volumes.

There is a still further problem with many rotary compressor systems inthat they commonly employ a pressure lowering valve to lower thepressure in the separator vessel down to the minimum pressure level soas to reduce the compression ratio of the compressor during unloadedoperation or when it is stopped. Some systems also operate under loadedand unloaded conditions cyclically and if, each time, it is operatedunloaded, the pressure lowering valve dumps pressure from the separatorvessel, then this amounts to a significant efficiency loss from thesystem. The operational mode of some systems is on a stop/start cyclebasis and again when the system is stopped, pressure is each time dumpedfrom the separator vessel resulting in a significant lack of efficiency.This of course also emphasises the problems discussed above with systemsusing pressure lowering valves.

The objective of the present invention is to provide a rotary compressorsystem, particularly for use in machinery of smaller capacity, whichwill reduce the complexity and size of the system package withoutsacrificing system performance characteristics.

Accordingly, the present invention provides a rotary compressor systemcharacterized by a pressure vessel acting as both a separator vessel anda compressed gas storage tank whereby compressed gas is supplied to anend user directly from said pressure vessel. Preferably said pressurevessel is relatively increased in size so that the pressure vessel alsoacts as an oil cooler. Conveniently, cooling fan means may be providedto pass cooling or ventilating air over said pressure vessel to increasecooling capacity.

It will of course be appreciated that by using only one pressure vesselfor both the separator and the storage tank functions, the overallpackage volume is significantly decreased. Moreover, while stilldecreasing the overall package volume, it is possible to use a "single"pressure vessel of substantially larger volume so that relativelyincreased volumes of oil can be used in the system. This also decreasesthe need for oil cooling capacity so that the oil can be adequatelycooled while in the pressure vessel without the need of using a separateoil cooler. Again, the ability of omitting a separate oil cooler allowsthe overall package volume to be decreased and simplifies the assemblyof the system.

In accordance with a further aspect of this invention, there is provideda compressor system comprising a rotary compressor unit arranged todeliver a mixture of compressed gas and oil entrained therein into apressure vessel, a drive motor coupled to said compressor unit to drivesaid unit, a filter element through which compressed gas passes fromsaid pressure vessel to an end user without passing through a separategas storage tank, oil return means for returning oil from said pressurevessel to the compressor unit, and means for preventing moisture buildup in said pressure vessel.

A problem exists when a single pressure vessel is used in replacement ofprior art arrangements employing both a separator vessel and a gasstorage tank. This problem is the possible condensation of moisture inthe oil as oil cools in the pressure vessel rather than in a separateoil cooler as in prior art systems. This situation is of courseexacerbated in systems which are operated infrequently whereby the oilis allowed to cool to a significant extent. Condensation build up in thepressure vessel will turn the oil into a form of mayonnaise which willmake the system unworkable.

To solve this problem the present invention provides means forpreventing moisture build up in the pressure vessel. This may beachieved in a first preferred embodiment by moisture removing means toremove moisture from gas flowing to the inlet zone of said compressorunit. In a second preferred embodiment, the means for preventingmoisture build up in the pressure vessel may comprise means for removingmoisture from the oil in the pressure vessel itself.

The difficulty with moisture in air being compressed is that themoisture condenses at high pressures and mixes with oil to form aconsistency like mayonnaise. Furthermore, in small capacity compressorsystems, compressed air consumption is usually variable so that heatrejection rates are difficult to control and prevention of moisturecondensation is therefore also difficult to achieve. This isparticularly difficult where the pressure vessel also acts as a coolersince the walls of the vessel are always cold. Moreover, in manyindustries, dry compressed air is required by the end user and inconsequence it is becoming increasingly more common for dryers to beprovided down stream of the compressor system so that the compressed gascan be dried. Thus, in one preferred aspect, the present invention aimsat providing a moisture removing means (dryer or the like) on thesuction side of the compressor unit thereby removing moisture beforecompression. While the use of dryers do involve the use of some energythereby lowering efficiencies somewhat, they are clearly not a penaltyin any industry already using dryers on the discharge side of thecompressor unit. Moreover the energy savings, by not having to blow downthe separator vessel, are believed likely to outweigh any inefficiencyinvolved in the use of a dryer on the suction side of the compressorunit.

In an alternative arrangement, in some systems, it may be possible forthe means for preventing moisture build up to be means to control thetemperature of the pressure vessel during system operation so that itwill run at a relatively hot temperature and that the temperature willbe built up rapidly at start up so that any condensed moisture is drivenoff in the compressed gas discharge.

Operating characteristics of the system are as follows. When thecompressor unit stops (control systems for all small capacity machinesis stop/start), a non return valve at the compressor inlet closes sothat air and oil cannot escape from the system. As a result, air and oilcannot escape from the system so that less power is consumed duringoperation.

A still further problem exists when a single pressure vessel is used inreplacement of prior art arrangements employing both a separator vesseland a gas storage tank. This problem is that the compressor unit muststart against full pressure in the pressure vessel which is not the casewith conventional systems using a separator vessel and a gas storagetank. With such conventional systems, the separator vessel is blown downto atmosphere before restarting the system but this cannot be done whena single large pressure vessel is used because too much compressed gaswould be lost. Screw compressor units have a fixed compression ratio sothat the output pressure is a fixed multiple of the inlet pressure. Forexample, if the compression ratio is eight and if the compressor unit isrestarted with say 6 bar inlet pressure (communicated from the pressurevessel), then the discharge pressure is 48 bars. It is possible withdirect drive between the motor and the compressor unit as isconventional in the prior art, that the aforementioned problem willcause the motor to stall thereby preventing restarting of the system. Ifstalling does not in fact occur, then at the very least, costly measuresof handling the momentary high pressures would be required. The presentinvention, in a preferred aspect also aims at providing a system whichwill solve the aforementioned difficulty.

In accordance with this aspect, the present invention aims to provide acompressor system which is capable of solving the aforementioned problemwhile using a single pressure vessel. Accordingly, the present inventionalso provides a rotary compressor system comprising a compressor unitarranged to deliver a mixture of compressed gas and oil entrainedtherein into a pressure vessel, a drive motor coupled to said screwcompressor unit to drive said unit, and regulator means enabling saidmotor to be started from a stopped condition with pressure of saidpressure vessel in an inlet region of said compressor unit.Conveniently, in the preferred embodiment, the regulator means comprisesa slip clutch coupling the motor to said compressor unit.

In a second preferred embodiment, the regulator means may comprise meansto control power supplied to the motor whereby the motor slowly buildsup to speed when restarted. In this case the motor may be directlycoupled to the compressor unit. The embodiment using a clutch meanscoupling is designed so as to allow slip in the drive coupling so thatgradual loading of the compressor unit occurs as it speeds up. Theclutch device may be a centrifugal type clutch but any other similardevice could also be used. Internal leakage in the compressor unitprevents build up of excessive pressure as the inlet is evacuated at lowspeed. The clutch device also limits maximum input torque therebyprotecting the compressor unit. The clutch device, at least in directcoupled machines (i.e. no belt or gear transmission), replaces thecoupling. Further, the peak start up amps drawn by the motor is reduced.

In accordance with a still further aspect of the present invention, asystem of the aforementioned type is proposed utilising a singlepressure vessel without any requirement of limiting the size of thepressure vessel so that a pressure differential can be quicklyestablished to create oil return flow to the compressor unit. Accordingto this aspect, the present invention proposes a rotary compressorsystem comprising a compressor unit arranged to deliver a mixture ofcompressed gas and oil entrained therein into a pressure vessel, a drivemotor coupled to said compressor unit to drive said unit, a minimumpressure valve arranged to maintain a minimum pressure in said pressurevessel during normal system operation, oil return means for returningoil from said pressure vessel to a zone of the screw compressor unithaving a first predetermined pressure during normal compressor systemoperation, valve means through which gas to be compressed flows to saidcompressor unit, said valve means being configured to establish a secondpredetermined pressure at said zone after start up of the compressorunit while still permitting gas flow into the compressor unit, saidsecond predetermined pressure being less than said first predeterminedpressure. Conveniently, a partial vacuum pressure is established at theinlet to the compressor unit whereby a pressure of up to (but preferablyslightly less than) one atmosphere is established at said zone where oilis reintroduced into the compressor unit whereby, after start up, anyincreased pressure in the pressure vessel causes a pressure differentialto create liquid flow from the pressure vessel to said zone. Thus, it isnot necessary to build the pressure in the vessel to a level above theminimum set by the minimum pressure valve before liquid flow to thecompressor unit begins. Conveniently, once the minimum pressure levelset by the minimum pressure valve is achieved in the pressure vessel,the valve means is adapted to open completely whereby the pressure atsaid zone is the first predetermined pressure.

In the aforementioned embodiments, the pressure within the pressurevessel is retained in the compressor unit and acts on the seals andvalves associated with the compressor unit. While this is not aninsurmountable problem, it would be preferable that this did not occur.

A preferred objective therefore of the present invention is to alsoprovide an arrangement in compressor systems of the aforementioned kindand a method of operating such systems which will avoid the prospect ofpressure being dumped cyclically from the system while at the same timeavoiding high pressure conditions within the compressor unit and makingstarting of the compressor unit easier.

According to this aspect, the present invention provides a compressorsystem comprising a rotary compressor unit with rotary compressionmeans, a motor driving said compressor unit, a pressure vessel receivingpressurised gas and oil discharged from a discharge end of saidcompressor unit with oil being returned from said vessel to an inletregion of said compressor unit, said system being characterized by firstvalve means controlling gas flow into the compressor unit, second valvemeans controlling flow of oil to the inlet region of the compressor unitfrom said pressure vessel, third valve means controlling gas/oildischarge from said compressor unit to flow to said vessel, and controlmeans arranged to control operation of first and second valve means andsaid motor whereby, in use, said first and second valve means are closedprior to cessation of rotation of said rotary compression means. Therotary compression means should complete at least one and preferablyseveral revolutions after the first and second valve means are closed soas to cause a vacuum in the inlet region of the compressor unit and soas most of the oil in the rotor region is discharged therefrom.Conveniently, the discharge volume (i.e. the gas containing volume ofthe compressor unit upstream of the non-return valve means anddownstream of the discharge point of intermeshing rotors) is selectedrelative to the intake volume of the compressor unit (i.e. the gascontaining volume downstream of the first valve means) so as to ensurean equilibrium pressure within the compressor unit when the valve meansare closed, that is, sufficiently low as to not inhibit restarting ofthe compressor unit. Conveniently the equilibrium pressure is about oneatmosphere but may be up to 2.5 to 3.0 atmospheres.

In accordance with the present invention, the rotary compressor unit maybe a screw compressor with intermeshing rotors forming the rotarycompression means or may be any other rotary compressor including vaneand scroll compressors.

Ensuring rotation of the rotary compression means after closure of thevalve means might be achieved by any one of a number of possible means.One means may be by simply selecting the inherent inertia of the rotarycompression means and the rotating components of the motor such thatwhen operation of the motor is discontinued, the inertia ensuressufficient numbers of revolutions prior to stopping to achieve thedesired vacuum conditions in the inlet region and the displacement ofliquid from the region of the rotary compression means. If the inherentinertia of the rotary compression means and rotating components of themotor is insufficient, then the system may include additional inertiasuch as a flywheel or the like to ensure rotation of the rotarycompression means for a sufficient period following closure of the valvemeans. In another possible configuration, the control means may bearranged so as to close the valve means first and allow the motor tooperate for a small but definite period after closure of the valvemeans.

At start up of a system of the aforementioned kind, where it is intendedto use differential pressure between the pressure vessel and thecompressor unit for recirculating liquid to the compressor unit, it isnecessary to build up pressure slowly to the minimum pressure level. Toachieve this, the first valve means is retained initially closed and asmall capacity gas line with a flow restrictor and valve means(preferably a non-return valve) directs gas flow downstream of the firstvalve means so that gas is slowly drawn into the inlet region of thecompressor unit. Once the minimum pressure is achieved, the first valvemeans is opened and normal operation follows. Moreover, if theequilibrium pressure is in fact a vacuum pressure in the compressor unitwhen the motor is stopped, then the gas bleed line may effectivelydeliver gas into this compressor unit to form an equilibrium pressure ofone atmosphere.

According to a further aspect of the present invention, there isprovided a method of operating a compressor system of the typecomprising a compressor unit with rotary compression means, a motordriving said rotary compression means, a pressure vessel receivingpressurised gas and oil from a discharge end of said compressor unitwith oil being returned from said vessel to an inlet region of saidcompressor unit, said method being characterized by closing first valvemeans controlling gas flow into the compressor unit and second valvemeans controlling oil flow back to the compressor unit, by apredetermined time prior to cessation of rotation of said rotarycompression means so as to create vacuum conditions in the inlet regionof said compressor unit and to displace oil from said rotors uponstopping of the motor.

By the arrangements and method discussed above, when it is desiredduring normal operation or general shut down of the system, to stopoperation of the compressor unit, the system permits normal pressures(i.e. one atmosphere or a pressure not greatly exceeding one atmosphere)to be maintained within the compressor unit thereby ensuring ease ofrestarting while at the same time pressure levels in the pressure vesselare maintained so that no losses occur that would affect efficiencylevels.

Several preferred embodiments will hereinafter be described withreference to the accompanying drawings, in which:

FIG. 1 is a schematic view of a first preferred embodiment;

FIGS. 2a and 2b are schematic views of two further preferredembodiments;

FIG. 3 is a schematic view of a still further preferred embodiment.

FIG. 4 is a schematic illustration of a still further preferredembodiment intended for use with smaller power motors; and

FIG. 5 is a schematic illustration similar to FIG. 4 modified forpossible use with larger powered motors.

With reference to FIG. 1, a compressor system 10 is schematicallyillustrated comprising a screw compressor unit 11 driven by a motor 12through a direct transmission which may include a centrifugal clutchdevice 13. The compressor unit 11 and motor 12 are conveniently mountedon a pressure vessel 14 so that compressed gas and entrained liquid isdischarged via line 15 directly into the vessel 14. A pool 16 of oil ismaintained in the bottom of the vessel 14 and is returned therefrom byline 17 via an oil filter 18 to an inlet region of the compressor unit11. Compressed gas with some oil droplets retained are discharged fromthe system direct to an end user via line 19 and a final filter 20. Thefilter element 20 may be mounted to the tank 14 with an arrangement forreturning oil collected in the filter element into the inlet region ofthe compressor unit 11. Alternatively, the filter element 20 might bemounted separately from the tank 14. Conveniently, the valving includesa non-return valve 23 which will allow air flow into the compressor unitduring operation but prevents compressed air and oil flow in the reversedirection if the oil compressor unit 11 stops. The valving 22 also mayinclude a solenoid valve 40 controlling gas inflow through line 41 intothe inlet zone of the compression unit 11. The solenoid valve 40 isactuated in response to signals from pressure sensing means PS1 and PS2adapted to sense pressure within the pressure vessel 14 as explainedhereinafter. Finally, a dryer 24 may be provided in the air flow passage25 into the compressor unit 11.

In normal operation, the suction air passes via line 25 to thecompressor inlet region, passing through the non-return valve 23. Oil isinjected and the air is compressed. The mixture of compressed air andoil is piped via line 15 to the pressure vessel 14 where most of the oilsettles by gravity to the pool 16 in the bottom of the vessel 14. Thecompressed gas (with small amounts of entrained oil droplets) leaves thevessel 12 via line 19 and is further cleaned by the fine oil filter 20before being discharged directly to an end user. The oil volume in thesystem can be quite large and has therefore a high thermal inertia. Itwill constantly cool by conduction with the walls of the vessel 14. Ifdesired, the vessel underside may be fitted with a fan 26 to increaseair flow levels over the belly of the vessel 14. At start up of thecompressor unit with the inlet valve closed, if the pressure in thevessel 14 is greater than a first predetermined (PS1) level defined by aminimum pressure valve (mpv) (for example 3.5 atmospheres) but less thanan upper level (PS2) (say 7 atmospheres) then the motor starts and thecompressor inlet opens. This is essentially normal operation. If thepressure is greater than the upper level (PS2), then the compressor willnot start if the pressure is less than (PS1) the inlet valve is closedbut the solenoid valve 40 opens. Flow through this valve is restrictedso that suction pressure is reduced to a partial vacuum in thecompressor inlet so that pressure differential allows oil flow alongline 17 to the compressor unit 11. When pressure in the vessel 14 getsabove (PS1) then the solenoid valve 40 closes and the inlet opens sothat normal air flow to the compressor unit is established. The solenoidvalve 40 is a normally closed valve and thereby line 41 is closed untilvalve 40 is opened as aforesaid.

FIGS. 2a and 2b illustrate arrangements similar to FIG. 1 but where thedryer 24 in the air inlet flow is omitted and moisture is removed fromthe pressure vessel 14 by a moisture removal means 35. In the case ofFIG. 2a, the means 35 comprises a line 27 removing oil from the pool 16,a regenerative heat exchanger 28, a hot oil sump 29, a heating device 30and a pump P. The heating device 30 is provided so that the oil in thesump 29 is sufficiently hot to evaporate moisture 31 out of the oil. Thepump P returns oil from the sump 29 via line 32 back into the pressurevessel 14. In doing so, it passes through the regenerative heatexchanger 28 to heat the oil leaving the pool 16 via line 27. In theembodiment of FIG. 2b, the means 35 comprises line 27, a coalescent typemoisture/oil separator 33, and pump P. The separator 33 removes moisturefrom the oil and the oil is returned via line 32 and pump P to thepressure vessel 14. In both cases, the flow rate of oil and the capacityof the pump P need only be relatively small so that upon operation,moisture is continuously removed.

The embodiments shown in FIGS. 1, 2a and 2b are relatively wasteful offloor space and to this extent, it might be desirable to arrange thepressure vessel 14 in an upright or vertical configuration as shown inFIG. 3. In this embodiment, items of a similar nature have been giventhe same reference numerals as in the earlier described embodiments. Inthis proposed embodiment the screw compressor unit 11 is at leastpartially mounted within the pressure vessel 14 and the discharge pipe15 therefrom discharges compressed gas and oil directly into the vessel14. It should of course be appreciated that it would be possible tomount the compressor unit 11 through the upright wall of the vessel 14with its axis horizontal or equally with the vessel 14 in a horizontalconfiguration, the compressor unit could be mounted horizontallyextending through an end wall of the vessel 14 or vertically extendingthrough a top horizontal wall section of the vessel 14. In theembodiment of FIG. 3, the motor 12 is directly coupled to the compressorunit 11 and a regulator 34 is provided to control the motor 12 on startup as indicated earlier in this specification. Such an arrangement couldalso be used in the embodiments of FIGS. 1, 2a and 2b if desired.

In this embodiment a dryer device 24 may be used (similar to FIG. 1) oralternatively, one of the moisture removal arrangements 35 disclosedwith reference to FIGS. 2a and 2b might be used instead of the dryer 24.As the wall of the pressure vessel 14 can become quite hot duringoperation, it is desirable to shield same and this may be done byplacing a concentric shield or wall 36 around same. The shield wall 36also defines an annular passage 37 through which cooling air might passto improve cooling effect.

In some compressor systems, it might be desired to use simply the heatof the pressure vessel 14 to prevent moisture condensing therein. Insuch systems, it would be necessary to ensure the system heats upquickly on start up and is maintained relatively hot when in operation.Thus, for example, it may be appropriate to provide a control system toprevent operation of the fan 26 on start up so that the system heats upquickly and runs for a predetermined period in a hot condition.Thereafter, the fan can be operated as needed to keep temperature of thevessel 14 within predetermined limits.

Referring now to FIG. 4 of the drawings, the system 10 comprises acompressor unit 11 with intermeshing rotors 42 driven by a motor 12. Themotor 12 is conveniently directly coupled to the compressor unit 11.Alternatively, a belt drive coupling may be useful in some circumstancesas the pulleys of the belt drive may be used to add inertia into therotating components as discussed in the following. The compressor unit11 has an air intake region 44 with first valve means 45 interposedbetween the region 44 and an air intake filter 60. The first valve means45 may be a two position solenoid valve which is normally closed butopened when air flow is desired. Any other form of valve capable ofeffecting a similar operation may also be utilised. Further, a line 46with a restriction 47 also permits air to flow into the inlet region 44via a non-return valve 48. The compressor unit 11 also has a dischargeregion 49 through which a compressed air and liquid mixture leaving therotors 42 is discharged. Flow through the discharge region 49 iscontrolled by valve means 50 which is arranged as close as possible tothe compressor unit 11 so as to limit the volume of the discharge region49. The valve means 50 may be a non-return valve (swing check or balltype) or may be a solenoid operated or equivalent type valve. In thelatter case, operation of the valve would be controlled by the controlsystem 51. A pressure vessel 14 is provided to receive the mixture ofcompressed gas and liquid leaving the compressor unit 11 via line 15.The liquid/compressed gas mixture undergoes a primary separation in thevessel 14 so as to maintain liquid 16 in the base of the vessel 14.

A liquid return line 17 is provided leading from the pool of liquid 16in the vessel 14 through a liquid oil filter 18 and second valve means52 eventually being delivered to the rotors 42 within the compressorunit 11. Again the valve means 52 may be a two position normally closedsolenoid valve but any other suitable valve means could be used. Liquidflow along line 17 depends upon a pressure differential existing betweenthe vessel 14 and the introduction point to the compressor unit 11. Ifthe arrangement is in accordance with FIGS. 1 to 3 then a cooling of theliquid returning to the compressor unit may not be necessary. A liquidcooler 53 may, however, also be employed as required. The compressed gasafter having most of the liquid removed from it within the vessel 14 isthen passed, via line 19 to a minimum pressure valve (mpv) and finalfilter element 20. After leaving the final filter element 20, the cleancompressed gas might be delivered directly to an end user or to a gasstorage tank 54 in a conventional system.

Finally, the control system 51 is provided controlling operation of thefirst valve means 45, the second valve means 52, and the motor 12. Thecontrol system, if required may also control operation of the valves 50and 48. The arrangement is such as to ensure the valve means 45 and 52are closed prior to the rotors 42 ceasing to rotate. The rotors 42should complete at least one and preferably several revolutions afterthe valves 45 and 52 are closed. This may be achieved by stopping themotor 12 a predetermined period of time after the valve means areclosed. Alternatively, the system may utilise inherent inertia to ensurethe rotors 42 continue to operate for a period of time after the motoris stopped. If necessary, extra inertia such as a flywheel 55 might beutilised.

It is also possible, to vary the volume of the intake region 44 relativeto the discharge region 49 so as to ensure the equilibrium pressurewithin the compressor unit (when stopped) does not exceed apredetermined level that would inhibit restarting of the system.Preferably this equilibrium pressure is about one atmosphere andpreferably does not exceed 2.5 to 3.0 atmospheres.

Reference will now be made to FIG. 5 of the annexed drawings. Likefeatures to the integers described above with reference to FIG. 4 havebeen given the same reference numerals. FIG. 5 represents a system foruse with larger powered motors and therefore capacity. Smallerhorsepower motors may be started by direct on-line connection to a powersupply, however, it is common practice for larger motors to be startedusing a star-delta starting means. In such systems the compressor unit11 is started under "star" regime (low motor torque). The first valvemeans 45 is closed causing vacuum conditions in the compressor inletregion 44. To prevent pressure build up in the small discharge volume 49(which would have the effect of increasing motor torque requirements), atwo position (normally closed) solenoid valve 56 is opened (via acontrol signal from the control 51) and vents the discharge zone 49 to avessel 57. The vessel 57 is connected via line 58 to the inlet region 44of the compressor unit. Line 58 may connect with line 46 upstream of therestrictor 47 or downstream of the restrictor 47 or valve 48 asillustrated in dotted lines 59. The valve 48 is shown as a non-returnvalve, however, it could also be formed as a solenoid valve or otherform of valve controlled by the control device 51. The vessel 57 may bequite small or if the volume of piping is sufficient, may be eliminatedaltogether. When the motor 12 switches to "delta" (high torque), thesolenoid valve 56 closes and the inlet or first valve means 45 opens. Itmay be possible for the start sequence to occur without the oil stopvalve (second valve means 52) opened in which case there would be noneed for the vessel 57. If this is not possible, then the valve means 52opens when the motor is operating in start regime and the vessel 57 alsocollects liquid. The vessel 57 drains liquid back to the compressorinlet over the first minutes of running. If desired, the vessel 57 maybe integrally formed with the inlet region and inlet filter.

It will of course be appreciated that the annexed drawings are schematicand do not represent any particular configuration or assembly of thevarious components. Any known arrangement of component parts couldequally be employed with the performance of the present invention.

I claim:
 1. A compressor system comprising a rotary compressor unit withrotary compression means, a motor arranged to drive said compressorunit, a pressure vessel receiving pressurised gas and oil dischargedfrom a discharge end of said compressor unit with oil being returnedfrom said pressure vessel to an inlet region of said compressor unit,said compressor system being characterized by first valve meanscontrolling gas flow into the compressor unit, second valve meanscontrolling flow of oil to the inlet region of the compressor unit fromsaid pressure vessel, third valve means controlling gas/oil dischargefrom said compressor unit to flow to said pressure vessel, and controlmeans to control operation of at least said first and second valve meansand said motor whereby, in use, said first and second valve means areclosed prior to cessation of rotation of said rotary compression means.2. A compressor system according to claim 1 wherein said rotarycompression means completes at least one revolution after the first andsecond valve means are closed whereby a vacuum or a partial vacuumcondition is established in the inlet region of the compressor unit. 3.A compressor system according to claim 1 or claim 2 wherein the systemincludes a discharge volume formed between a discharge point of therotary compression means and the third valve means, and an intake gasvolume formed downstream of the first valve means, the relative sizes ofsaid discharge volume and said intake gas volume, being selected so asto ensure an equilibrium pressure is established within the compressorunit when the first and second valve means are closed, that issufficiently low as to not inhibit restarting of the compressor unit. 4.A compressor system according to claim 3 wherein the equilibriumpressure is less than 3.0 atmospheres.
 5. A compressor system accordingto claim 1 further including a secondary gas flow means to the inletregion of the compressor unit, the secondary gas flow means including aflow restrictor and fourth valve means and being arranged to direct gasflow into said inlet region downstream of said first valve means.
 6. Acompressor system according to claim 5 further including a minimumpressure valve whereby, upon start up, gas flow initially flows throughsaid secondary gas flow means with said first valve means beingmaintained closed until pressure in said pressure vessel reaches aminimum pressure defined by said minimum pressure valve, whereupon saidfirst valve means is opened.
 7. A method of operating a compressorsystem of the type comprising a compressor unit with rotary compressionmeans, a motor driving said rotary compression means, a pressure vesselreceiving pressurised gas and oil from a discharge end of saidcompressor unit with oil being returned from said vessel to an inletregion of said compressor unit, said method being characterized byclosing first valve means controlling gas flow into the compressor unitand second valve means controlling oil flow back to the compressor unit,by a predetermined time prior to cessation of rotation of said rotarycompression means so as to create vacuum conditions in the inlet regionof said compressor unit and to displace oil from said rotors uponstopping of the motor.